This invention relates to a surface electrode which can be used medically to receive and transmit biosignals emanating from a body or to apply low level electrical signals to the body.
Medical surface monitoring electrodes are well known. For example, one such electrode consists of a silver-plated eyelet which is housed within a recessed plastic element or cup. A snap fastener stud is located on the outside to the plastic element and acts as a means of connecting the external circuitry to the electrode. In this type of electrode there is a sponge which has been impregnated with an electrolytic gel and which is located within the plastic electrode housing so that when in use the sponge serves as a conductive bridge between the eyelet and the patient's skin. The electrolytic gel enhances the conductivity of the skin and ensures good electrical contact between the patient and the metal sensor. Since the electrode system must have good contact with the skin, the present technology provides that the plastic housing incorporating the eyelet sensor, with its conductive gel, be attached to a disc of open cell plastic foam or microporous tape, which is coated on its underside with a medical-grade contact adhesive. This resilient adhesive disc serves to attach the system to, and hold it on the patient's skin. Finally, for storage purposes a cap is placed over the rigid plastic element in order to isolate the electrode from the atmosphere and thus prevent the drying-out of the conductive gel, which is water based.
Electrodes of the type just described have proven quite reliable in establishing an electrical connection to the patient but associated with them are several disadvantages. Firstly the design incorporates many components which render the electrode somewhat complex in assembly and therefore relatively expensive to manufacture. Secondly it has a large profile, covers a considerable skin area and lacks flexibility. This rigidity of the element, or housing, can give rise to skin abrasion and irritation and pull on the connecting lead affects the sensor and can give rise to motion artefact signals.
Recently a simpler, less expensive, electrode design has become available and it employs a metal foil which acts as both an electrode sensor and as a means of connection to the external circuitry. The connection to the metal foil is via an exposed tab of foil which in general practice is grabbed by a small alligator clip.
In this newer type a solid, adhesive, hydrogel serves both as the electrolyte and the adhesive means to the skin. The system has electrode flexibility, the desired low profile and it conforms well to body contours. Since the system dispenses with the conventional disc of adhesive backing, the overall electrode area is small. This electrode design is simple and less expensive to manufacture.
Although it is an improvement on the metal eyelet electrode system this metal foil electrode has a major disadvantage in that the foils generally used are not silver, because of expense, and therefore have relatively poor electrical properties. The cost of this electrode is still not sufficiently low.